Method of casting hollow metal balls

ABSTRACT

A cast steel ball having a centrally located spherical void and an uninterrupted outer surface is provided for use in mills used to pulverize material. The ball is produced by a centrifugal casting method in which the axis of rotation of the ball is constantly changed during solidification of the molten steel. A mold arrangement made of a chill material and having no risers is provided in order that the outer surface of the ball is uninterrupted and of substantially the same grannular structure throughout.

United States Patent 1191 Schmuck et a1.

[ METHOD OF CASTING HOLLOW METAL BALLS [75] lnventors: James E. Schmuck,Tempe, Ariz.;

Walter C. Troy, National City, Calif.

[73] Assignee: AMSTED Industries, llncorporated,

Chicago, 111.

22 Filed: Dec. 1, 1971 211 App]. No.: 203,593

[52] U.S. Cl. 164/116 [51] Int. Cl B22d'l3/06 [58] Field of Search164/116,115,114; 425/429, 435

[56] References Cited UNITED STATES PATENTS 2,325,019 7/1943 Rubissow164/114 X 3,104,423 9/1963 Kemper 425/429 Y 3,528,133 9/1970 Morse425/435 1 July 17, 1973 2,222,266 11/1940 Rubissow 164/115 3,614,97610/1971 Bolling et al 164/114 X FOREIGN PATENTS OR APPLICATIONS1,020,076 2/1966 Great Britain 164/1 16 Primary ExaminerRobert D.Baldwin Attorney-Walter L. Schlegal, Jr. et a1.

[57] ABSTRACT A cast steel ball having a centrally located sphericalvoid and an uninterrupted outer surface is provided for use in millsused to pulverize material. The ball is produced by a centrifugalcasting method in which the axis of rotation of the ball is constantlychanged during solidification of the molten steel. A mold arrangementmade ofa chill material and having no risers is provided in order thatthe outer surface of the ball is uninterrupted and of substantially thesame grannular structure throughout.

3 Claims, 6 Drawing Figures Patented July 17, 1973 3,746,073

3 Sheets-Sheet 1 fizz/91:51.5

Patented July 17, 1973 3,746,073

3 Sheets-Sheet 2 m g 68 hm.

METHOD OF CASTING HOLLOW METAL BALLS This patent relates to a cast metalball having a void located at its approximate geometric center and to amethod and apparatus used in making the ball.

Metal balls have been used in mills for pulverizing coal used in firingboilers. As contact is made between the ball and the material to becrushed a given amount of the balls outer surface is worn away. A hardabrasion resistant outer skin is therefore advantageous to prolong theuseful life of the ball. Furthermore, it is desirable to have balls witha relatively smooth outer surface so that during rotation of the millthe material to be crushed is located between the outer surface of theball and a surface of the mill. Voids in the outer surface of the ballallow material to locate therein requiring added rotation to pulverizethe material and possibly the passage of particles that are larger thandesired.

These balls have traditionally been made by forging. Limitations of theforging process, however, prevent the use of high alloy materials thathave desirable abrasion resistant qualities. It has therefore beennecessary to accept high wear rates for forged balls.

A cast ball has recently come into use. The balls are cast out of highalloy materials in sand molds which create a good grain structure. It isdifficult, however, to feed a cast ball in a mold. Risers must berelatively large thereby creating, in the area of feed, a surface havinga lower abrasion resistant quality than an adjacent surface contactingthe mold. During use of such balls certain sections of the outer surfacewill wear faster than other sections. Such differential wear is highlyundesirable.

It is further desirable for the balls to have a centrally located void.This void has been accomplished to a degree by the use of a coresuspended by a cross-shaped support in the mold. These supports,however, leave either holes in the outer surface of the ball or highwear surface areas. Furthermore, after the casting solidifies the centerof the balls contain the material of the core which is generally sand.During use of the balls, the outer skin is worn away thereby opening thesand filled cavity to its surroundings and allowing the sand to be mixedwith the coal or the substance being pulverized. Such mixing is highlyundesirable. Movement of the core prior to or during casting alsocreates problems. A ball having an off centered void has un unfavorabledynamic balance which to a degree affects the opera- 7 tion of the mill.

BRIEF DESCRIPTION OF THE INVENTION The above problems and others havebeen solved by applicants by providing an alloy iron ball having acentrally located spherical void and an uninterrupted chillsolidifiedbody. In producing these balls, molten metal is poured into a sphericalcavity located in a stationary graphite mold. After a given period oftime the mold is slowly rotated on a horizontal axis that correspondswith the geometric center of the ball. As the metal inside the cavitysolidifies a shrinkage void develops inside the ball. Since the ball isbeing rotated, the molten metal within the ball continuously washes overthe inside of the solidifying skull causing the skull to increase inthickness without the formation of discontinuities thereby locating thevoid toward the center of the ball. When the skull is thick enough toallow safe handling of the ball with melt inside, the ball is removedfrom the mold and centrifugally spun by a method which causes randomlocation of the spin axis. By constantly changing the spin axis, theshrinkage void becomes centrally located within the ball.

To accomplish the random location of the spin axis the partiallysolidified ball may be placed inside of a cylindrical drum which isrotated on a horizontal axis. The drum is spun at an angular velocitysufficient to centrifugally cast the melt in the ball. To change theaxis of rotation of the ball, the drum is either oscillated or rotatedon a vertical axis. The oscillational or rotational movement is resistedby the angular momentum of the spinning ball causing a certain amount ofskidding between the ball and the surface of the drum and acorresponding changing of the axis of rotation of the ball. Suchskidding may be aided by the presence of graphite particles. Thesolidified ball is removed from the drum and heat treated in anappropriate manner.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. '1 is a schematic sectional viewof a mold arrangement used in casting metal balls taken on thecenterline of the casting cavity.

FIG. 2 is a side elevational view of a machine used to cast metal ballsand illustrates the mold arrangement of FIG. 1 in an operationalposition.

FIG. 3 is a top plan view of the machine illustrated in FIG. 2.

FIG. 4 is a schematic side elevational view, partially in section, of anapparatus used for spinning a metal ball during solidification.

FIG. 5 is a sectional view of a cast metal ball taken on a line whichcorresponds to line A--A in FIG. 2 and embodying features of theinvention.

FIG. 6 is a view of another cast metal ball sectioned similar to the oneillustrated in FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS A mold arrangement 10,schematically illustrated in FIG. 1, comprises mating upper and lowerblocks 12 and 14 made of graphite or another chill material defining acavity 16. A passage 18 is located in upper block 12 for venting airduring casting. Cured thermosetting sand 19 may be used to preventmolten metal from entering passage 18 and permit the passage of air.

Cavity 16 is filled with molten metal from the bottom through a passage20, which may be lined with clay, cured thermosetting sand or the like.The lining material is chosen with the idea of controlling the cut-offtime, i.e., the time it takes the metal to freeze within passage 20during casting. A runner 22 made of cured thermosetting sand has avertical passage 23 in comm unication with a horizontal passage 24 whichcommunicates with passage 20. It is desired that the horizontal passage24 extends beyond passage 20 for several inches to achieve favorablecasting conditions. The passage 23 extends above the upper portion ofcavity 16 in order that molten metal introduced into passage 23 willcompletely fill cavity 16 as a result of the force of gravity. Runner 22may be detachably connected to mold arrangement 10 in any one of anumber of known ways (not shown).

A machine 26 (FIG. 2 and 3) may be used to rotate the mold arrangement10 about an axis A-A that passes through the geometric center of cavity16. A frame 28 has sections 30 and 32 respectively around blocks 12 and14. Spaced L shaped members 34 are secured to each section 30, 32 andcontact an outer surface of each block 12,14. The sections 30 and 32along with the L shaped members 34 are drawn toward one another by bolts36 and nuts 38 to tightly engage the mold blocks 12 and 14.

A pair of spaced circular shafts 40 and 42 in axial alignment areconnected to section 32. Each shaft 40, 42 is mounted on a pair ofrollers 44 secured to machine 26 proximate mold arrangement 10 in amanner to permit easy rotation of the shafts about axis AA. Circularretaining plates 46 are secured to each shaft 40,42 on opposite sides ofeach pair of rollers 44 to prevent axial movement of the shafts. Acrossbar 48 is connected to the end of shaft 40 for rotating the moldarrangement 10 byhand.

An electric variable speed motor 50 on machine 26 is connected to a gearreducer 52 in a known manner. A standard roller chain 54 interconnectssprocket wheels 56 and 57 which are respectively connected to a shaft ongear reducer 52 and shaft 42 in a manner to transmit rotational movementtherebetween. Shaft 42, and accordingly mold arrangement 10, may berotated by motor 50 throughout a range of angular velocities.

ln operation, cavity 16 is sprayed with a known mold wash. The two moldblocks 12 and 14 are then mated and clamped together by frame 28. Bar 48is then turned by hand to position mold arrangement 10 so that pouringpassage 20 is vertical and located at the bottom of cavity 16. Runner 22is then secured to mold arrangement 10. Molten metal is poured intopassage 23 of the stationary mold arrangement 10. The cavity 16 shouldbe filled in about 20 seconds.

At the completion of the pour it is desirable to seal passage 20 as soonas possible. A known slide gate (not shown), may be used for thispurpose or chill rods made of steel may be inserted into passage 23 tospeed up the freezing of molten metal therein. If the chill rods areused, after a period of time, mold 10 may be slowly rotated by hand todump excessive metal out of runner 22. With a 12% inch diameter ball,mold 10 may be hand rotated about 30 seconds after cavity 16 is filled.

After the ingate 22 is removed, mold 10 may be slowly rotated at anangular velocity of from to 20 rpm by motor 50. This rotation serves toprevent internal separation of the solidifying metal by maintaining thesolidifying skin at a temperature at which the molten metal will bond orweld. The ball should never be left at one location long enough to allowthe skin to chill to a temperature which is too low to permit bonding orwelding of the molten metal. Mold arrangement should be rotated for asufficient period of time to allow the molten metal to form a skin thatis thick enough to allow safe handling of the cast ball with moltenmetal inside.

After approximately 3 minutes of hand rotation of mold 10 the solidifiedskull ofa 12% inch diameter ball is generally strong enough to permitremoval of the partially liquid ball so that it may be centrifugallyspun. The transfer of the ball to the centrifugal casting machine mustbe completed in less than 45 seconds because of the possibility ofdeveloping an internal separation caused by the ball being located inone position for a period of time. An apparatus for performing thecentrifugal casting of the ball on a randomized spin axis is illustratedin FIG. 4.

As previously noted, the ball is moved from mold arrangement 10 ofmachine 26 to a drum 58 as quickly as possible. The ball should not bepermitted to remain in a static position during transfer. The drum 58 isrotated on an axis B-B which passes through a shaft 60 mounted in knownpillow blocks 62. A known motor pulley arrangement 64 is used to rotateshaft 60 and drum 58 at an angular velocity suitable for the ball beingcast, e.g., 150 rpm for a 12% inch diameter ball. Higher angularvelocities are needed for smaller diameter balls and lower ones forlarger balls.

As previously noted, during this stage of solidification of the moltenmetal within the ball, the axis of rotation of the ball should beconstantly changed. This change may be accomplished in drum 58 bymounting drum 58 and the motor-pulley arrangement 64 on a turntable 66that may be moved a number of degrees upon an axis that is angularrelative to axis B--B. Where possible, 360 rotation of turntable 66 ispreferred and may be achieved by mounting the turntable on rollers 68and rotating the turntable by hand or known power means. Multi-axialspinning of the ball is predicated upon the fact that skidding betweenthe ball and drum 58 occurs when turntable 66 rotates or oscil lates.The skidding may be aided by adding a solid lubricant, such as graphite,to drum 58. When the lubricant is added and spin speeds are within thespecific range, the ball will spin at right angles to the spin axis B--Bof drum 58. Since the spin axis B-B is con stantly changing due tomovement of turntable 66 the spin axis of the ball will also beconstantly Changing.

The ball should be spun in drum 58 until the outer surface of the ballhas reached a temperature which indicates the completion of thesolidification process. This temperature may be determined by an opticalpyrometer or other means. The temperature, of course, will varydepending upon the given metal used. The solidified ball may thereafterbe removed from drum 58 I and heat treated.

It is necessary to maintain certain variables during casting withingiven limits. The temperature of the molten metal during casting iscritical. If the metal is too cold pouring wrinkles and laps will appearon the outer surface of the ball. If the molten metal as poured is toohot, longer time intervals are required in order to accomplish thedesired solidification. in brief the pouring temperature must beselected in accord with casting characteristics of the particularmaterial, as is well known in the art.

A cross-section of a typical cast ball 76 is shown in FIG. 5. Theperiphery or outer surface 78 of ball 76 is preferably smooth, tough andabrasion resistant. The shrinkage void 80 is substantially spherical butmay have a slightly rough or erose inner surface 81. The skin 82 of theball between the inner and outer surfaces 81 and 78 is preferablyuniform in hardness, from outside to inside but may be slightly lessabrasion resistant toward void 80.

It should be appreciated that the presence at void 80 is an essentialaffirmative element in configurational design of ball 76. It is not anecessary evil related to shrinkage but is rather a definite benefit.Elastic reactions are different in a solid ball as compared with ahollow ball both in service and during the occurence of transientstresses in manufacture. For these reasons the void requires engineeringattention, particularly with respect to its centralization. Ill-centeredvoids will both disturb the residual state of stress and impair thedynamic balance of ball 76.

Balls 83 having a slightly elongated center void 84, please note FIG. 6,may be manufactured with the arrangement illustrated in FIGS. 1, 2 and3. Even though a ball 83 is not as acceptable as a ball 76, they may beused successfully in many applications. A ball 83 with a slightlyelongated or oval shaped but centrally located void 84 may be producedin cavity 16 by rotating mold after the passage 20 has frozen at anangular velocity of from 10-200 rpm with best results occuring in the100 to 150 rpm range. Angular velocities above 150 rpm tend to elongatethe shrinkage void 84 along the axis A-A of rotation while angularvelocities lower than 100 rpm tend to form voids having extremely jaggedinternal surfaces and possibly may form a plurality of shrinkage voids.

Rotational movement of mold arrangement 10 during the casting ofa ball83 must exist for a sufficient period of time in order to permitsolidification of all the molten metal' within the ball. Such timeperiod, of course, will depend upon molten metal temperature at pouring,the diameter of the cavity and the temperature of blocks 12 and 14. Uponsolidification of the molten metal, balls similar to the one illustratedin FIG. 6 may be heat treated.

Regardless which method is used to produce the ball, the molten metaltherein must be constantly moved. Prolonged static periods will onlyresult in improper welding of the molten material to a cold inner skin.It has been determined that such improper welding will result ininternal separation cracks which have undesirable effects on the radialheat flow within the ball. Portions of the ball lying outboard from aseparation crack will heat more rapidly than other portions of the ballbecause of their partial isolation by a heat flow interface. Thedifferent rates of heating leads to extremely high axial compressionstresses on the portion of the ball outboard from a separation crackwith the usual consequence of severing the ball on a diametrical planenormal to the separation crack. It is, therefore, desirable to have ahomogeneous skin with a uniform grain structure at any given radialdistance from the centroid.

What is claimed is:

l. A method of casting spherical metal articles with a centrally locatedspherical void comprising the steps of: filling a spherical cavity of amold with molten metal; then permitting a portion of the molten metal tosolidify at the surface of the cavity forming a selfvsustaininghomogeneous skull; then quickly removing the partially solidifiedarticle from the cavity and placing the partially solidified article ina drum disposed on a horizontal axis; then rotating the drum about thehorizontal axis while simultaneously rotating the drum about a verticalaxis disposed outside of the drum so that the article is spinning on aconstantly changing axis; and continuing such rotation until the articlehas solidified.

2. The method of claim 1 wherein the mold is rotated during the step ofpartially solidifying the molten metal at the surface of the cavity. j

3. The method of claim 1, and including the step of lubricating theinternal surface of the drum prior to placing the partially solidifiedarticle therein.

1. A method of casting spherical metal articles with a centrally locatedspherical void comprising the steps of: filling a spherical cavity of amold with molten metal; then permitting a portion of the molten metal tosolidify at the surface of the cavity forming a self-sustaininghomogeneous skull; then quickly removing the partially solidifiedarticle from the cavity and placing the partially solidified article ina drum disposed on a horizontal axis; then rotating the drum about thehorizontal axis while simultaneously rotating the drum about a verticalaxis disposed outside of the drum so that the article is spinning on aconstantly changing axis; and continuing such rotation until the articlehas solidified.
 2. The method of claim 1 wherein the mold is rotatedduring the step of partially solidifying the molten metal at the surfaceof the cavity.
 3. The method of claim 1, and including the step oflubricating the internal surface of the drum prior to placing thepartially solidified article therein.